1. Field of the Invention
The present invention relates to an input monitoring system for an optical amplifying repeater, and more particularly to an input monitoring system for an optical amplifying repeater, which includes an optical fiber amplifier for amplifying an optical signal.
2. Description of the Related Art
FIG. 14 shows a basic structure of an optical amplifying repeater having a function of ALC (Automatic Level Control), for which an object of the present invention is present.
In FIG. 14, reference numeral “1” is an optical fiber transmission path. An optical fiber amplifier 2 is formed of an optical fiber, which is doped with rare-earth material, such as Erbium (Er).
In this example, a principle of the optical fiber amplifier 2 is the same as that of a laser. The Er doped fiber generates a predetermined Fermi energy level, at which strong adsorption occurs. Accordingly, when electrons in an optical fiber are excited to the high Fermi energy level and thus the energy level of Er is reversed, a light having a wave-length corresponding to the difference between the high Fermi energy level and the low Fermi level is inputted and causes induced scattering in the optical fiber amplifier 2. As the result, an amplified optical signal is output from the optical fiber amplifier 2.
In FIG. 14, reference numeral “3” is a coupler. For exciting electrons in an optical fiber as described above, an exciting light outputted from a laser diode 4 is supplied to an optical fiber amplifier 2 via the coupler 3.
The amplified optical signal outputted from the optical fiber amplifier 2 branches in a coupler 5, and the signal branching is inputted to a photo diode unit 6. A photo diode provided in the photo diode unit 6 converts the amplified optical signal to an corresponding electrical signal. In the photo diode unit 6, the electrical signal outputted from the photo diode is further compared with a predetermined reference signal, and the difference between the electrical signal and the predetermined reference signal is supplied to an amplifier 7 to control a bias current for the laser diode 4. Thus, the output of the laser diode 4 is automatically level-controlled.
In this example, ALC (Automatic Level Control) means to control the optical output signal sent from the laser diode 4 for exciting the optical fiber amplifier 2 according to the input signal level, and control so as to keep the level of the output signal to constant by changing an amplifying rate of the optical fiber amplifier 2.
On the other hand, the repeater employs a structure for monitoring input signal levels to detect faults generated on the optical fiber transmission paths. FIG. 15 shows a conventional structural example to monitor the input signal level. In FIG. 15, a coupler 10 is provided on input side of the optical fiber amplifier 2. An optical input signal branches at the coupler 10. Then, the signal branching is inputted to a photo diode unit 11 for monitoring the input level.
The photo diode unit 11 for monitoring input level converts the optical input signal, which branched in the coupler 10, to an electrical signal to judge the input level of the signal. However, in the structure, when the level of the optical input signal is low, the level of the optical signal inputted to the photo diode 11 becomes more lower level, according to the branching in the coupler 10.
Accordingly, errors become larger on detecting level of the optical input signal, which is converted to an electrical signal in the photo diode unit 11 and monitored. Reversely, if a branching ratio to the photo diode unit 11 becomes larger, a signal component, which is inputted to the optical fiber amplifier 2, becomes smaller, thus making deterioration of SN ratio (Signal-to-Noise ratio) larger. A branching ratio, such as 20:1, is selected as usual.
Further, an optical fiber for transmitting the signal sent from a signal light source is connected to the input side of the optical fiber amplifier 2 shown in FIG. 15. The optical fiber has the property, of reflecting slightly an incoming optical signal to the reversed direction in an optical signal path, which is called so as Rayleigh scattering. The residual excited light, which is not used to amplify, leaks at the input side of the optical amplifier. Then, the leaked light goes back to the optical amplifier according to the Rayleigh scattering inside of the optical fiber for transmission, and enters to the photo diode for monitoring the input level with the signal light. Therefore, the input monitor value is not equal to an input power of the signal light, as shown in FIG. 17. Accordingly, the input power cannot be normally measured.
When the exciting light is supplied in the same direction as that of the signal light, namely by a forward excitation, the residual exciting light, which is not used to amplify, leaks at the output side of the optical fiber amplifier 2. Therefore, the leaked light is delivered to the next optical amplifier via an optical fiber for transmission. The residual excited light enters to the photo diode for monitoring the input level of the next amplifier with the signal light. Accordingly, it is also impossible to normally measure the input power at the next optical amplifier.
FIG. 16 shows the other conventional structural example for monitoring the input signal level. As described above, the automatic level controller (ALC) controls the optical output signal of the laser diode 4 to change according to the input signal level. The photo diode 13 monitors the optical output of the laser diode 4, so that it becomes possible to equivalently find an optical input signal level.
However, in this structure, if the level of the optical input signal is large, light-emission of the laser diode 4 becomes small to reduce an amplifying ratio of the optical fiber amplifier 2.
Accordingly, in a structure shown in FIG. 16, the level of the exciting light, which is inputted to the photo diode 13 for monitoring, and branches at the coupler 3, becomes smaller, whereas error due to the result of monitoring becomes larger.
As described above, in the conventional input monitoring system of the optical amplifying repeater, either of cases where the level of the optical input signal is low, that is, as shown in FIG. 15, and the level of the optical input signal is high, that is, as shown in FIG. 9, makes error in monitoring larger.
Therefore, if the optical input signal level varies in a wide range, it is impossible to accurately monitor the levels.